1. Field of the Invention
The present invention relates to robotic graspers or hands and, more particularly, to a robust compliant underactuated mechanical grasper and method of manufacturing the same.
2. Brief Description of the Related Art
After years of experimenting with complex, fully-articulated anthropomorphic hands, researchers have begun to embrace the idea that much of the functionality of a hand can be retained by careful selection of joint coupling schemes, reducing the number of actuators and the overall complexity of the grasping mechanism. Many of these grippers are ‘underactuated’, having fewer actuators than degrees-of-freedom. These types of graspers, grippers or hands have also been referred to as ‘adaptive’ or ‘selfadaptable’. Other simplified hands have fixed-motion coupling between joints, reducing the overall degrees-of-freedom of the mechanism. These two classes of simplified grippers can be easier to control, much lighter, and less expensive than their fully-actuated counterparts.
The very nature of unstructured environments precludes full utilization of a complex, fully-actuated hand. In order to appropriately use the added degrees of actuation, an accurate model of the task environment is necessary. A gripper with a reduced number of actuators is not only simpler to use, it is more appropriate based on the quality of information available for the unstructured grasping task.
The joint coupling necessary to allow for underactuation is often accomplished through compliance in the manipulator structure. Compliance is perhaps the simplest way to allow for coupling between joints without enforcing the fixed-motion coupling relationship inherent with gear or linkage couplings. Compliant couplings are a simple way to allow a joint to passively deflect without causing a fixed-motion proportional change in the joints to which it is coupled.
Compliant underactuated grippers show particular promise for use in unstructured environments, where object properties are not known a priori and sensing is prone to error. Finger compliance allows the gripper to passively conform to a wide range of objects while minimizing contact forces. Passive compliance offers additional benefits, particularly in impacts, where control loop delays may lead to poor control of contact forces. See D. E. Whitney, “Quasi-static assembly of compliantly supported rigid parts,” Journal Dyn. Syst. Measurement Control 104, pp. 65-77, 1982 and J. M. Schimmels and S. Huang, “A passive mechanism that improves robotic positioning through compliance and constraint,” Robotics Comput.-Integr. Manuf. 12 (1), 65-71, 1996. Compliance can also lower implementation costs by reducing the sensing and actuation required for the gripper.
A number of underactuated and fixed-motion coupled robotic hands have been proposed. Those prior devices include the devices described in the following publications: [1] M. Higashimori, M. Kaneko, A. Namiki, M. Ishikawa, “Design of the 100G Capturing Robot Based on Dynamic Preshaping,” The International Journal of Robotics Research, vol. 24 (9), pp. 743-753, 2005; [2] W. T. Townsend, “The BarrettHand Grasper—Programmably Flexible Part Handling and Assembly,” Industrial Robot—An International Journal, vol 10 (3), pp. 181-188, 2000; [3] M. Rakic, “Multifingered Robot Hand with Self-Adaptability,” Robotics and Computer Integrated Manufacturing, vol. 5(2/3), pp. 269-276, 1989; [4] J. Butterfass, G. Hirzinger, S. Knoch, H. Liu, “DLR's Multisensory Articulated Hand Part I: Hard- and Software Architecture,” Proceedings of the 1998 IEEE International Conference on Robotics and Automation, pp. 2081-2086, 1998; [5] J. Butterfass, M. Grebenstein, H. Liu, G. Hirzinger, “DLR-Hand II: Next Generation of a Dextrous Robot Hand,” Proceedings of the 2001 IEEE International Conference on Robotics and Automation, pp. 109-114, 2001; [6] A. Edsinger-Gonzales, “Design of a Compliant and Force Sensing Hand for a Humanoid Robot,” Proceedings of the 2004 International Conference on Humanoid Manipulation and Grasping (IMG04), 2004; [7] J. Crisman, C. Kanojia, I. Zeid, “Graspar: A Flexible, Easily Controllable Robotic Hand,” IEEE Robotics and Automation Magazine, pp. 32-38, June 1996; [8] S. Hirose and Y. Umetani, “The Development of Soft Gripper for the Versatile Robot Hand,” Mechanism and Machine Theory, vol. 13, pp. 351-359, 1978; [9]T. Laliberte, L. Birglen, C. Gosselin, “Underactuation in Robotic Grasping Hands,”Machine Intelligence & Robotic Control, vol. 4 (3) pp. 1-11, 2002; [10] J. Ueda, Y. Ishida, M. Kondo, T. Ogasawara, “Development of the NAIST-Hand with Vision-based Tactile Fingertip Sensor,” Proceedings of the 2005 IEEE International Conference on Robotics and Automation, pp. 2343-2348, 2005; [11] E. Torres-Jara, “Obrero: A platform for sensitive manipulation,” Proceedings of the 2005 IEEE-RAS International Conference on Humanoid Robots, pp. 327-332, 2005; [12] C. S. Lovchik, M. A. Diftler, “The Robonaut Hand: A Dexterous Robot Hand for Space,” Proceedings of the 1999 IEEE International Conference on Robotics and Automation, pp. 907-912, 1999; [13] K. DeLaurentis, C. Mavroidis, “Mechanical design of a shape memory allow actuated prosthetic hand,” Technology and Health Care, vol. 10, pp. 91-106, 2002; [14] D. Caldwell, N. Tsagarakis, ““Soft” grasping using a dextrous hand,” Industrial Robot: An International Journal vol. 27 (3), pp. 194-199, 2000; [15] R. Crowder, V. Dubey, P. Chappell, D. Whatley, “A Multi-Fingered End Effector for Unstructured Environments,” Proceedings of the 1999 IEEE International Conference on Robotics and Automation, pp. 3038-3043, 1999; [16] M. C. Carrozza, C. Suppo, F. Sebastiani, B. Massa, F. Vecchi, R. Lazzarini, M. R. Cutkosky, P. Dario, “The SPRING Hand: Development of a self-Adaptive Prosthesis for Restoring Natural Grasping,” Autonomous Robots 16, pp. 125-141, 2004; [17] N. Dechev, W. Cleghorn, S. Naumann, “Multiple finger, passive adaptive grasp prosthetic hand,” Mechanism and Machine Theory 36, pp. 1157-1173, 2001; and [18] F. Lotti, P. Tiezzi, G. Vassura, L. Biagiotti, G. Palli, C. Melchiorri, “Development of UB Hand 3: Early Results,” Proceedings of the 2005 IEEE International Conference on Robotics and Automation, pp. 4499-4504, 2005. Table I provides an overview of some of the features of those underactuated and fixed-motion coupled robotic hands.
TABLE 1UNDERACTUATED AND FIXED-MOTION COUPLED ROBOT HANDSPitchPitchCoupling schemejoints peractuators(*indicates compliant couplingSource of complianceHand# fingersfingerper finger{circumflex over ( )}indicates adaptive mechanism)Coupling ratioand/or adaptability100G [1]22½prox:*:distunknowntendon routing, spring-loaded jointsBarrett [2]321prox:{circumflex over ( )}:dist(3:4)“TorqueSwitch” differentialBelgrade/USC [3]4 + 13 + 0½ + 1(prox;med;dist) + (prox;dist)(~9;8;7)rocker arm coupling of fingersDLR I and432med;dist(1;1)noneII [4, 5]Domo [6]331prox;med*:dist(1;1:passive)unactuated compliant distal jointGraspar [7]331prox:{circumflex over ( )}:med:{circumflex over ( )}:dist(~5:4.2:2.9)tendon differential mechanismHirose [8]210 ½prox:(all):distal(55:::28:::10:::1)tendon routingLaval 10-DOF [9]33⅓prox:{circumflex over ( )}:med:{circumflex over ( )}:distunknownadaptive linkage mechanismNAIST [10]3 + 13 + 32 + 2(med;dist) + (med;dist)(1;1.15)noneObrero [11]321prox:*:dist(4:3)series elastic actuationRobonaut [12]2 + 2 + 13 + 3 + 22 + 1 + 2(med;dist) + (prox;med;dist) + 0(1;1) + (1;1;1) + 0compliant connector, no adaptabilityRutgers [13]4 + 13 + 32 + 2med:distunknowntendon routingSalford [14]4 + 13 + 32 + 3(med;dist) + 0unknownnoneSDM [15]221(prox:*:dist)(4.5:1)tendon routing, joints made ofspringsSouthampton [15]331prox:{circumflex over ( )}:med:{circumflex over ( )}:distunknowndifferential unitSPRING [16]2 + 13 + 2⅓ + ⅓(prox:*:med:*:dist) + (prox:*:dist)(2.9:1.6:1)series elastic actuationTBM [17]4 + 13 + 21 + 1(prox;med;dist) + (prox;dist)(~2;1;1) + (~2;1)noneUB III [18]2 + 2 + 13 + 3 + 33 + 2 + 20 + (med:*:dist) + (med:*:dist)(~6:7)tendon routing, joints made ofsprings
An ‘underactuated’ hand has fewer actuators than degrees-of-freedom, and therefore demonstrates adaptive behavior. In these hands, motion of the distal links can continue after contact on the coupled proximal links occurs, allowing the finger to passively adapt to the object shape. In a ‘fixed-motion coupled’ hand, each actuator controls a single degree-of-freedom, and the mechanism has no ‘adaptability’ (final column). In these hands, motion of one joint always results in a proportional motion of the joint(s) coupled to it. In the same way, if contact occurs on one joint fixing its position, all coupled joints are thereby fixed.
In Table I, the ‘# fingers’ column gives the number of fingers of each different type used in the hand, separated by ‘+’. Cases where two types are given indicate that some number of identical fingers and one thumb are used in the design. Cases where three types are given mean that two different finger designs are used in addition to a thumb. For example, the Robonaut hand incorporates two “grasping” fingers, two “dexterous” fingers, and a thumb. The second column indicates the number of ‘pitch’ joints per finger, leaving out ‘yaw’ and ‘roll’ joints, if any exist. Entries correspond to the data in the ‘# fingers’ column. For the Robonaut hand, the grasping and dexterous fingers and thumb have three pitch joints each. The next column corresponds to the number of actuators per finger that control the pitch joints. Note that the degree of underactuation ranges from a single actuator for twenty joints (Hirose's “Soft Gripper”) to twelve actuators for fifteen joints (UB III hand). The coupling scheme is indicated in the next column. ‘Prox’ indicates the proximal joint (nearest to the base), ‘med’ is the medial joint (for three phalanx fingers), and ‘dist’ is the distal joint (farthest from the base). A ‘:*:’ between two joints indicates that the coupling between the two joints is compliant, such as those hands with joints made of springs. A ‘:^:’ between two joints indicates that the coupling between the two joints is based on a mechanism that allows for decoupling. The BarrettHand, for example, achieves this effect by means of a “TorqueSwitch” differential gear mechanism that actively decouples the two joints once contact has been made on the inner link and a preset torque limit has been reached. A ‘;’ between joints indicates that the coupling is fixed-motion, and therefore has no adaptability. The next column indicates the coupling ratio (prox:med:dist) between the joints. For a finger with some method of adaptability, this ratio is the relative angular motion between joints when the finger is freely actuated (i.e. no external contact). For Hirose's “Soft Gripper,” every third value is given. The final column indicates the method by which the hand is passively compliant and/or adaptive, if at all.
Grasping and manipulating objects in unstructured environments, where object properties are not known a priori and sensing is prone to error, is one of the central challenges in robotics. The uncertainty in the relationship between the object and gripper makes it difficult to control contact forces and establish a successful grasp. One approach to dealing with this uncertainty is through compliance, so that positioning errors do not result in large forces and the grasper conforms to the object. Compliance has most often been implemented through control of manipulator impedance, based on active use of joint sensors for position, velocity and force.
While designing durable robots is rarely addressed in robotics research, it is essential in industrial, space, and military applications. Examples include iRobot's “PackBot”, University of Minnesota's “Scout” family of launchable robots, and MIT manipulator arms for the NASA/JPL. Pathfinder and Surveyor Mars missions. This durability would expand the type of experimental tasks that can be reasonably attempted and speed implementation due to the reduced need for careful validation of programs.
Unintended contact that often occurs in unstructured grasping tasks can result in large contact forces unless the gripper is compliant. This contact can occur due to sensing uncertainty in unstructured environments, but can also happen in laboratory experiments, particularly in the debugging phase. Researchers are often reluctant to risk crashes with expensive multi-degree-of-freedom robot hands, so implementations must be carefully validated and experimental scope must be limited.
Compliance conveys two key advantages for robotic grasping: adaptability and robustness. The present invention takes advantage of the adaptability inherent with compliance and enhances it by incorporating further adaptability in the form of underactuation. An underactuated hand has fewer actuators than degrees of freedom, and therefore demonstrates adaptive behavior. In these hands, the transmission design allows motion of other joints to continue after contact occurs on a coupled link, allowing the hand to passively adapt to the object shape during finger closure.
Additionally, many complicated robotic hands suffer from drawbacks of being unreliable and difficult to use. Many simpler robotic hands suffer from a drawback of being aesthetically unappealing. The present invention provides a reliable robotic hand that is relatively simple to use and may be implemented with a molding process that may produce aesthetically acceptable appearance.